Progressive power lenses typically comprise a far-vision region having one refractive power, a near-vision region having a different refractive power, and an intermediate progressive region.
According to a common practice, semi-finished progressive lens blanks are provided by lens manufacturer to prescription labs. Generally a semi-finished progressive lens blank comprises a front progressive surface and a back spherical surface (“standard semi-finished lens blank”). A standard semi-finished lens blank having suitable optical characteristics is then selected based on a prescription. The back spherical surface is finally machined and polished by the prescription lab (based on the base curve) so as to obtain a sphero-torical surface complying with the prescription. A progressive power lens complying with the prescription is thus obtained.